The present invention generally relates an apparatus for growing semiconductor crystals and more particularly to an apparatus for growing a multi-component compound semiconductor crystal.
Compound semiconductor crystals such as gallium arsenide (GaAs) or indium phosphorus (InP) are used in various laser diodes for telecommunication purposes and the like because of their characteristic band structure which facilitates the emission of light by direct transition. It should be noted that, in laser diodes, the oscillation wavelength is determined by the band gap which is pertinent to the material of the compound semiconductor forming the active layer of the laser diode. Thus, the wavelength of the light that the laser diode produces is generally fixed. However, in the field of telecommunication in particular, there arises rather frequently a case in which other wavelengths are desirable. In order to design the laser diode which can oscillate at such a desired wavelength, one has to choose the material for the active layer so that the material has a proper band gap and the desired oscillation wavelength.
In choosing the compound semiconductor material, one has to keep in mind that the material should have a proper matching of the lattice constant with respect to the substrate. When mismatching occurs, mechanical stress develops which eventually deteriorates the operation of the semiconductor laser or ultimately destroys the structure of the laser diode. In order to solve this problem, use of a buffer layer or a graded layer structure has been proposed so as to relax the stress as much as possible. However, such a solution is complex and a provides only a limited success.
One possible solution to this problem is the use of multicomponent compound semiconductor material having three or more components for the substrate. When using GaAs or InP for the substrate, there is no degree of freedom in choosing the composition of the substrate since the composition of these materials is determined by the stoichiometry of Ga and As or In and P. On the other hand, when one uses a ternary compound semiconductor material such as (Ga, In)As, one can continuously change the composition of the substrate by changing the ratio of Ga and In. Thus, by choosing the composition such that the lattice constant of the substrate substantially matches with that of the active layer, one can eliminate all the complexity pertinent to the device which combines the desired compound semiconductor with the binary compound semiconductor substrate. In order to realize such a device, however, one has to establish a technique to grow a bulk crystal of the compound semiconductor material having three or more components.
Such a multi-component compound semiconductor crystal is grown from a melt of a compound semiconductor material during an epitaxial phase. FIG. 1 shows a prior art apparatus used for growing a ternary compound semiconductor crystal having a composition of In.sub.0.53 Ga.sub.0.47 As from a melt. Referring to the drawing, a melt 1 of indium gallium arsenide (InGaAs) having a composition of In:Ga:As=0.790:0.040:0.170 in mole ratio (represented hereinafter as In.sub.0.790 Ga.sub.0.040 As.sub.0.170) for example, is loaded in a crucible 2 and maintained at a temperature of 840.degree. C. The melt 1 is then cooled to 780.degree. C. Responsive to the cooling, an InGaAs crystal 3 having the desired composition of In.sub.0.53 Ga.sub.0.47 As is grown on a substrate or seed crystal 4 at a tip of a rod 5.
In such a primitive method of growing, there is a problem in that the composition of the melt 1 is changed with time as illustrated in FIG. 2. Referring to the drawing, the composition of the melt 1 in equilibrium with the crystal 3 having the aforementioned composition at the beginning of the crystalization is represented by a point A. Upon cooling, the crystal 3 grows while maintaining equilibrium with the melt 1. When the melt 1 and the crystal 3 are in equilibrium, gallium (Ga) is preferentially incorporated into the crystal 3 as compared with indium (In) because of the non-uniform partitioning of elements between the melt and the crystal. Thus, there appears a depletion of Ga in the melt 1 as well as a depletion of arsenic (As) which is crystalized together with Ga. With further growth of the crystal, this depletion of Ga and As in the melt 1 becomes conspicuous. Further, the partition coefficient itself is also changed with temperature.
FIG. 2 shows an example of the compositional change of the melt 1 upon cooling. In the drawing, a tie line I shows an ideal path of the compositional change in which the composition of the crystal 3 in equilibrium with the melt 1 is maintained at In.sub.0.53 Ga.sub.0.47 As during the growth. Further, the actual path of the composition of the melt 1 is represented by another tie line I' represented by a broken line. It can be seen that the composition of the melt 1 is deviated significantly from the ideal tie line I as a result of the concentration of Ga in the crystal 3 which causes t he depletion of Ga in the melt 1. When such a depletion of Ga occurs in the melt, the composition of the crystal grown from the melt is changed continuously while it is growing. In the final stage of crystalization, the InGaAs crystal can even have a composition of In.sub.0.7 Ga.sub.0.3 As. In other words, there appears a compositional inhomogeneity in the obtained InGaAs crystal 3. Such an inhomogenous crystal cannot be used for the substrate of the semiconductor devices.
In order to compensate such a depletion of Ga in the melt, Japanese Laid-open Patent Application NO. 62-18706 describes an apparatus shown in FIG. 3. In FIG. 3, these parts corresponding to those parts in FIG. 1 are given identical reference numerals and the description thereof will be omitted.
Referring to FIG. 3, the crucible 2 comprises a side wall portion 2a made of carbon so as to act as an electrode and a bottom part 2b made of boron nitride. Further, there is provided a source compound chip 6 of gallium arsenide (GaAs) embedded in the bottom part 2b. Furthermore, there is provided a carbon electrode 7 behind the source compound chip 6 and a d.c. voltage from a voltage source 8 is applied across the side wall 2a and the electrode 7 such that a current i flows from the side wall 2a to the electrode 7 through the melt 1 as well as through the source compound chip 6 as indicated by a broken line. In this construction, the current i passing through the source compound chip 6 produces a Joule heat which causes a local heating of the chip 6. At the same time, the current causes Peltier heating at a surface 6a of the source compound chip 6 in contact with the melt 1. In one example, a current with a density of about 16 A/cm.sup.2 is flowed through the melt 1. As a result, Ga and As in the source compound chip 6 is dissolved into the melt 1 and the compensation to the depletion of Ga and As in the melt 1 is achieved. Further, Ga and As thus dissolved into the melt 1 is transported to the crystal 3 at the tip of the rod 5 by electromigration along a reversed path of the current.
This prior art apparatus, however, has a problem in that the supply of Ga and As to the melt 1 tends to become unstable as the current flows through the source compound chip 6 itself which is gradually dissolved into the melt 1. More specifically, there is a tendency that a number of minute projections and depressions are formed on the surface 6a as schematically shown in FIG. 3 in correspondence to the local path of the electrical current in the chip 6. Such projections and depressions enhances the concentration of current and the state of the surface 6a is changed continuously over time. As a result, the current flowing across the electrode 2a and the electrode 7 fluctuates over time which is difficult to control even with an elaborate control system. Thus, there appears a fluctuation in the supply of Ga and As to the melt 1 which results in the compositional inhomogeneity in the obtained crystal 3. Further, this prior art apparatus has another problem of unstable electromigration of Ga and As because of the unstable electrical current.
FIG. 4 shows another prior art apparatus. In the drawing, these parts corresponding to those parts in FIG. 1 are given identical reference numerals and the description thereof will be omitted.
Referring to FIG. 4, the crystal 3 of InGaAs is held on a carbon electrode plate 3a at the top of the crucible 2 so as to be in contact with the melt 1. Further, the source compound chip 6 is held in a carbon holder 7a and the d.c. voltage source 8 is connected across the electrode plate 3a and the carbon holder 7a. In this construction, the current flowing through the melt 1 causes Peltier cooling at a surface 3b of the crystal 3 and the growth of the crystal is controlled by the current causing the Peltier cooling. Responsive to the growth of the crystal 3, there appears the depletion of Ga in the melt 1 particularly in the vicinity of the crystal 3. This depletion of Ga in the melt is compensated by the source compound chip 6 which releases Ga and As to the melt 1. Thus, Ga and As dissolved into the melt 1 is transported to the vicinity of the crystal 3 mainly by diffusion. It should be noted that the electromigration does not contribute significantly to the transport of these dissolved species as the current path avoids the source compound chip 6. However, such a diffusion is not an efficient process for supplying the depleted Ga and As to the melt 1 in the vicinity of the crystal 3 and there is a tendency that the composition of the crystal 3 is deviated from the ideal composition. Further, as the current i passes through the crystal 3 itself, the surface 3b of the crystal 3 tends to become rough and there appears the fluctuation of the current i similarly to the case of the apparatus of FIG. 1. Such a fluctuation of the current i causes fluctuation in the Peltier cooling at the surface 3b which in turn invites inhomogenous growth and compositional inhomogeneity of the obtained crystal 3. It is of course possible to facilitate the transport of Ga and As from the source compound chip 6 to the part of the melt 1 surrounding the crystal 3 by designing the holder 7a such that current passes through the source compound chip 6 directly and Ga and As dissolved into the melt 1 is transported by the electromigration. However, such a construction inevitably causes unstable flow of the electrical current which in turn invites the unstable supply of Ga and As as already described with reference to the apparatus of FIG. 3.